The Text tool's Size setting in the Tool Options palette is expressed in points where the calculation is relative to the pixel per inch resolution of your monitor. Most monitors in today are designed to be 96 pixels to the inch. Points are a unit of measurement where 72.27 points are equal to one inch. However, the computer world has changed the measurement to 72 points per inch. Thus a 24-point font is roughly equivalent to 1/3 of an inch.
Be aware that this is relative to the number of pixels per inch. The calculation to go from size to display fontsize is :
fontsize = (Size x Monitor-pixels-per-inch) / 72
Using the above example of Size set to 24-point:
fontsize = (24 x 96) / 72 = 2304 / 72 = 32
since 32 / 96 = 1/3 then relative to the monitor specifications, the fontsize is 1/3 of an inch.
But since our images also carry a resolution, (for example 200 pixels per inch) and printers may have yet a different (usually higher) resolution, a typical printer resolution is 300 dpi or 600 dpi or greater.
In order for a 1/3 inch size font on a 300 dpi image, then the value should be 100. Which means to correctly calculate the appropriate size for points, the above algorithm should replace monitor pixels per inch with image pixels per inch or image resolution.
For example, if your image dimensions were one inch wide by one inch high, and the image resolution were 100 dpi, printing this image on a 300 x 300 dpi printer will produce an image roughly one-third of an inch square.